With regard to a sputtering-based film formation device incorporating a magnetron, it has been known that configuration of the cathode of a magnetron can be classified to the balanced type and unbalanced one in terms of the distribution of magnetic fluxes.
According to the balanced type magnetic flux designing, a magnetic field is produced so as to restrict plasma strictly locally to the surface of a target which is placed on the surface of a cathode. To achieve this mode of magnetic field setting, a magnetron includes a plurality of permanent magnets, and adjusts the intensities of individual magnets such that magnetic fluxes emanating from N poles of the magnets pass through a space adjacent to the target and converge almost totally to S poles of the magnets, namely, a closed magnetic flux circuit is formed through the space. According to this mode of operation, it is possible to restrict plasma strictly close to the surface of the target.
In contrast with this, according to the unbalanced type magnetic flux designing, a magnetron includes a plurality of permanent magnets, and adjust the intensities of individual magnets such that one part of magnetic fluxes forms a closed circuit as does a magnetron prepared according to the balanced type magnetic flux designing, while the other part is radiated in a three-dimensional space to be dispersed diffusely (see URL: http://www.kobelco.co.jp/p109/pvd/ubms.htm).
These two types of magnetic flux designing have their own advantages and disadvantages in terms of film formation. According to the balanced type designing, it is possible to restrict plasma necessary for film formation strictly close to the surface of cathode which allows the high speed formation of a film.
In contrast, the cathode of a magnetron sputtering device arranged to produce a magnetic field evoking an unbalanced distribution of magnetic fluxes allows plasma not only to be distributed close to the surface of cathode but also to be dispersed diffusely towards the surface of a substrate which is placed opposite to the cathode and upon which a film is to be formed, which allows the more vigorous spread of ions and radicals or reactive elementary particles in plasma onto the surface of the substrate than is possible with a corresponding cathode arranged according to the balanced type designing, thereby achieving the more enhanced elicitation of reactions and growth of crystals upon the surface of the substrate, although the cathode in question does not allow the formation of a film to proceed as fast as the comparable cathode based on the balanced type designing.
Thanks to the above merits, so it has been thought, the cathode of a magnetron sputtering device arranged to produce an unbalanced magnetic field can be applied to a substrate constituted of a heat-labile material which will reject a film formation process necessarily accompanied by high temperature, and will allow a well-crystallized film to be formed at a lower temperature than is possible with a comparable cathode prepared according to the balanced type magnetic flux designing.
When a film is to be formed on a substrate, the most appropriate mode of magnetic flux arrangement should be chosen according to a given purpose of the film formation. To meet such a requirement, it is ideal to prepare two devices one operating exclusively according to the balanced type magnetic flux designing and the other to the unbalanced counterpart, and to choose either one according to a given requirement of film formation. However, the sputtering device is so expensive that the problem under study is generally solved by removing the cathode of a single existing device and adjusting parameters of the cathode so that it allows the generation of magnetic fluxes whose distribution is most appropriate for a given purpose of film formation, that is, the magnetic flux distribution is changed from one mode to the other or vice versa according to a given purpose of film formation.
This switching operation will be described below with reference to attached drawings taking, as an example, a case where the cathode of a magnetron sputtering device which has been set to the balanced mode is altered to give a magnetic field to evoke an unbalanced distribution of magnetic fluxes. Needless to say, the reverse case may happen as often.
Referring to FIG. 1, magnetic fluxes (not shown) for restricting plasma (not shown) to the surface of a material target (4) are generated by a magnet assembly (7) comprising a plurality of permanent magnets (6) placed behind a backing plate (5) which holds the material target (4). The magnetic fluxes pass through the backing plate (5) and material target (4) to form a magnetic field close to the surface of material target. The balanced magnetic field arrangement (FIG. 1) can be achieved by mounting, to the device, a magnet assembly where the intensities of individual magnets have been adjusted to allow magnetic fluxes emanating from N poles of the magnets to converge nearly totally onto S poles of the magnets, namely, a closed magnetic flux circuit to be formed between the two poles.
Let's assume that the above device should be altered to provide an unbalanced distribution of magnetic fluxes. First, the magnet assembly (7) comprising magnets (6) as shown in FIG. 1 which has been set to produce a balanced distribution of magnetic fluxes is removed, and it is exchanged for another magnet assembly (11) as shown in FIG. 2 where the intensities of individual magnets of the assembly have been adjusted so as to cause a part of magnetic fluxes emanating from N poles of the magnets not to return to S poles of the magnets but to be dispersed diffusely in a three-dimensional space (see URL: http://www.kobelco.co.jp/p109/pvd/ubms.htm).
Alternately, switching the distribution of magnetic fluxes from the balanced mode to the unbalanced one may occur as follows. A magnet assembly which has been set to give a balanced distribution of magnetic fluxes is removed and disassembled. Into the magnet assembly thus disassembled, a magnetic modifier (8) such as a ferromagnetic body may be inserted as shown in FIG. 3 so as to modify the convergence state of magnetic fluxes to convert thereby the balanced mode of magnetic flux distribution into the unbalanced one.
However, the actual process necessary for the conversion is not so simple. Let's assume that a device set to give a balanced distribution of magnetic fluxes as shown in FIG. 1 should be altered to give an unbalanced distribution as shown in FIGS. 2 and 3. Cooling water in contact with the backing plate (5) is purged; cooling water pipes (9) and a power cable (10) are removed; a balanced type magnet assembly (7) (bulky and heavy) is removed, and an unbalanced type magnet assembly (11) is mounted instead, or the balanced type assembly is modified by adding a modifier such as a ferromagnetic body thereto so as to establish an unbalanced distribution of magnetic fluxes. In the latter case, during the installment of the modified magnet assembly, the cooling water pipes (9) and power cable (10) must be reconnected. All these procedures are so complicated and require such a hard labor that completion of them will take several hours.
Above all, adjustment of the magnet assembly to give a desired configuration of magnetic field requires experience and technique so rich and high that even a technician skilled in the work does not feel ease at the work. In addition, when modification of a magnet assembly consists of inserting a modifier constituted of a ferromagnetic substance into a gap at one end of a magnet, insertion of the modifier itself is often difficult actually because there is not enough room at the ends of a magnet. Thus, modification of a magnet assembly by inserting a modifier at one end of an appropriate magnet is often infeasible.
In magnetron sputtering, the quality of a film formed thereby is very important because it seriously affects all the subsequent processes leading to the production of a final product. In this respect, choice of the type of magnetic field design, and adjustment of a magnet assembly in accordance with the design are very important. Thus, it is necessary to separately use two kinds of cathodes operating on two different modes, one to give a magnetic field capable of evoking a balanced distribution of magnetic fluxes and the other a magnetic field capable of evoking an unbalanced distribution of magnetic fluxes, depending on the material of a film to be formed and purpose of the product.
Specifically, when the process for film formation may tolerate a comparatively high temperature, but must proceed at a high speed, a cathode configured to give a balanced mode magnetic field should be adopted. On the contrary, when the process for film formation does not tolerate a high temperature, a cathode configured to give an unbalanced mode magnetic field should be used at a low temperature, even though the speed with which film is formed will be reduced, because then it will be possible to produce a film possessed of high quality crystals.
Requirement for switching from the balanced mode operation to the unbalanced one or vice versa occurs at a considerably high frequency in research and production sites, and thus the switching operation requiring several hours as mentioned above has been a heavy burden for those involved in the operation.
Even if it is decided, for example, to select a cathode arrangement that gives an unbalanced magnetic field, it is still necessary to further adjust the radiation of magnetic fluxes into a three-dimensional space (by altering the intensities of individual magnets, their distribution, and size and shape of a ferromagnetic body) depending on the material of a film to be formed so as to give an optimum result for that particular material. In such an optimization work, it is often necessary to switch between the balanced mode operation and unbalanced one many times for a single operation, which will further contribute to the prolongation of time necessary for switching, and thereby add to the heavy burden.
Furthermore, adjusting the radiation of magnetic fluxes into a three-dimensional space (by altering the intensities of individual magnets, their distribution, and size and shape of a ferromagnetic body) requires highly advanced technical knowledge and experience, and is not easily achievable.
Still further, if it is required to convert configuration of the cathode from the balanced mode to the unbalanced one and the conversion is performed by inserting a ferromagnetic body into a gap between a backing plate (5) and an appropriate magnet (6), a case is often encountered where there is no enough room between the backing plate (5) and the magnet (6), because usually the cathode has been designed to allow magnetic fluxes to efficiently converge onto the surface of a material target. Because of this, even if it is required to alter the configuration of an existing magnetron sputtering device so as to give a magnetic field matching the property of a given material, adjustment of the radiation of magnetic fluxes into a three-dimensional space is often limited within a certain range. Thus, it is often impossible to modify the configuration of cathode to such a degree as to evoke a magnetic field that provides a desired unbalanced distribution of magnetic fluxes. Particularly even if it is required to produce an unbalanced magnetic field to enable low temperature sputtering, the requirement may not be satisfied in some cases. Thus, demand is acute for a remedy to drastically solve the problem.
It has been known, to smoothly achieve low temperature sputtering, it is more advantageous to employ a so-called dual magnetron sputtering device incorporating two units of magnetrons which allows the rapid formation of a film at a low temperature, as compared with an ordinary magnetron sputtering device based on a single unit of magnetron which will require full scale operation within its capacity to achieve the same purpose. In the dual magnetron sputtering device, the same problem as described above in relation to an ordinary magnetron sputtering device is also observed: selection of the balanced type magnetic flux design or unbalanced one should be made according to the property of material, and switching from one type to the other requires much labor and skill, and imposes a heavy burden to those involved in the operation. The problem also remains to be solved.
Recently, demand is manifest for film forming sputtering which allows the formation of a film made of an inorganic material having a high melting point over a substrate constituted of an organic polymer sheet having a low melting point. To meet the demand, particularly to provide sputtering allowing the formation of such a film at a low temperature, it is necessary to provide a device designed to give an unbalanced distribution of magnetic fluxes, as well as a film formation method enabling the formation of a film at a low temperature. It is often difficult to meet the demand even for a dual magnetron sputtering device which has been designed to produce an unbalanced distribution of magnetic fluxes. Thus, it is often difficult even for a dual magnetron sputtering device to meet all the requirements imposed by the recently developed widely varied materials and their combinations. Solutions to these problems are strongly wanted.
The background art relating to the first aspect of the invention has been described above. Next, the prior art relating to the second aspect of the invention, that is, a dual magnetron sputtering device will be described.
First, a conventional dual magnetron sputtering device will be described with reference to FIG. 9.
Referring to FIG. 9, two units of magnetrons (13) are placed opposite to respective material targets (14). Magnetic flux lines (15) are arranged abreast crosswise to form a closed circuit emanating from N poles and ending in S poles to take a balanced mode of magnetic flux distribution. In the device, two targets (14), (14) of the two units of magnetrons are placed to be abreast on the same plane.
An alternate electric power having a desired waveform and frequency is applied between the two units of magnetrons (13) to elicit sputtering. Then, a material constituting target (14) is turned into material vapor (17) which is released into the chamber of the device to deposit on a substrate (16) to form a film there as intended, at a higher speed than is possible with a single magnetron sputtering device.
However, with a conventional dual magnetron device, it is difficult to form a film with well-oriented crystals while keeping the target at a low temperature, or more typically not heating the target in any way, particularly when the material consists of an inorganic material having a high melting point. For example, even if it was tried to prepare a film of titanium oxide which has a photocatalytic activity on a substrate kept unheated, the trial has never met success.
Therefore, it has been impossible to form, without heating, a film of titanium oxide having a photocatalytic activity on a substrate made of a material having a low melting point represented by polyethylene terephthalate which, on account of restrictions imposed by the physical property of the material, requires non-heating film formation.
The merit of dual magnetron sputtering includes the rapid formation of a film. However, if the formation of a film of titanium dioxide which will normally have an excellent photolytic activity occurs on a glass substrate with heating at a deposition speed of 20 namometer or more per minute, the resulting film of titanium dioxide will not exhibit an excellent photocatalytic activity characteristic with titanium dioxide.
To obtain a substrate coated with a film of titanium dioxide exhibiting an excellent photocatalytic activity characteristic with titanium dioxide, it is necessary not only to select an appropriate material for the substrate, but also to reduce the speed of film formation, which spoils the merit inherent to dual magnetron sputtering.